A fuel cell utilizing a proton (cation) exchange membrane (PEM) as the electrolyte and employing a direct feed fuel such as methanol, ethanol, dimethoxymethane, trimethoxymethane, etc., and oxygen/air as the oxidant has the capability to replace batteries in small, portable applications. Direct methanol proton exchange membrane fuel cells (DMPEMFC's) are of particular interest for such applications. At the present time, the performance level of DMPEMFC's is almost high enough that small cells of this type can be competitive with primary lithium batteries in terms of size and weight. Such fuel cells have several advantages over lithium batteries including (a) the potential for much lighter weight and greater compactness, especially for long-duration operating times, (b) simpler "recharge" involving only the addition of fuel rather than battery replacement and (c) elimination of disposal issues (quite expensive for lithium batteries) and the need for storage of batteries.
The DMPEMFC is also a potentially attractive power source for vehicles and other low to medium power applications such as auxiliary power supplies and lawn mowers. Benefits to be derived from using DMPEMFC's as power sources include dramatic reductions in emissions of air pollutants, reduction in the nation's dependence on imported petroleum since methanol can be made from indigenous fuels such as coal and natural gas and also from renewable sources such as wood and biomass, and an overall increase in energy efficiency. Since liquid methanol as a fuel has a much higher energy density, it avoids the difficulties and hazards associated with the handling of gaseous reactants such as hydrogen. Consequently, DMPEMEC's have the potential for use in vehicles, particularly in California and the Northeast where there are initiatives for low or zero-emission vehicles.
One drawback to fuel cells which employ direct feed fuels, particularly DMPEMFC's, is that that proton (cation) exchange membranes do not totally prevent the so-called "crossover" of fuel through the membrane. The term "crossover" refers to the undesirable transport of fuel through the membrane from the fuel electrode or anode side to the oxygen electrode or cathode side of the fuel cell.
The fuel crossover diminishes cell performance for two primary reasons. Firstly, the transported fuel cannot react electrochemically on the anode side and, therefore, contributes directly to a loss of fuel efficiency (effectively a fuel leak). Secondly, the transported fuel interacts with the cathode i.e., the oxygen electrode, and lowers its operating potential and hence the overall cell voltage. The reduction of cell voltage lowers specific cell power output and also reduces the overall efficiency. Therefore, it is especially desirable to provide a cation exchange membrane for use in a fuel cell which has a low fuel crossover rate.